Building and Construction - Sizing Shear Walls

Author

Sizing Shear Walls

Wayne Whitney

2005-12-12, 1:21 pm

Hello,

I'm trying to understand how shear walls are sized for seismic forces
according to Chapter 16 of the 1997 UBC. I'm interested in the case
of a one or two story light frame residence with cripple walls.

Do I understand correctly that the shear force on the lowest diaphragm
will equal the base shear? The seismic forces are distributed
vertically among the diaphragms according to the weighted average of
(weight * height), but each shear wall transfers the forces on the
diaphragm above to the diaphragm below, so at the lowest diaphragm the
total accumulated shear force equals the base shear?

If so, then for a rectangular configuration each of the two cripple
walls in a given direction direction need to be able to transfer half
of the base shear from the diaphragm above to the foundation. For an
example, if the base shear is 18000 lbs and a cripple wall is 20.5
feet long with only 9 feet of braced panels, each braced panel would
have to be rated for a shear of 1000 lbs/ft?

Thanks, Wayne

Bob Morrison

2005-12-12, 2:21 pm

In a previous post Wayne Whitney wrote...
> I'm trying to understand how shear walls are sized for seismic forces
> according to Chapter 16 of the 1997 UBC. I'm interested in the case
> of a one or two story light frame residence with cripple walls.
>
> Do I understand correctly that the shear force on the lowest diaphragm
> will equal the base shear? The seismic forces are distributed
> vertically among the diaphragms according to the weighted average of
> (weight * height), but each shear wall transfers the forces on the
> diaphragm above to the diaphragm below, so at the lowest diaphragm the
> total accumulated shear force equals the base shear?
>
> If so, then for a rectangular configuration each of the two cripple
> walls in a given direction direction need to be able to transfer half
> of the base shear from the diaphragm above to the foundation. For an
> example, if the base shear is 18000 lbs and a cripple wall is 20.5
> feet long with only 9 feet of braced panels, each braced panel would
> have to be rated for a shear of 1000 lbs/ft?
>

Wayne:

You are correct in your assumptions. Where it gets difficult is when
there is more than one shear line and the shear walls above don't line up
with the shear walls below.

Also, you need to make sure that your hold-down forces are cumulative from
the floors above.

BTW, most jurisdictions are now using IBC2003 or IRC2003. Here's a link
to an older design guide that might be of some help:

http://www.pathnet.org/sp.asp?id=1442

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA

Wayne Whitney

2005-12-12, 4:21 pm

On 2005-12-12, Bob Morrison <bob@_remove_rlmorrisonengr.com> wrote:

> BTW, most jurisdictions are now using IBC2003 or IRC2003. Here's a link
> to an older design guide that might be of some help:

Thanks for your kind reply and the link to the design guide, I was
looking for something like that but couldn't find it. I live in
Berkeley, CA, and my understanding is that the current building code
is the 2001 CBC, based on the the 1997 UBC, with some revisions. I'm
trying to understand the seismic design for adding a partial second
story to my single story house with partial basement. I will hire an
engineer before applying for a permit, but I'd like to understand how
everything is done.

A few additional questions: with the partial basement, is the top of
the concrete retaining wall (6"-12" above grade) considered the base?
I'd like to have several windows in the cripple walls around the
basement, which reduces the possible length of braced panels. Are
there any preengineered products like Simpson Strong Wall and Simpson
Steel Strong Wall which are suitable for cripple walls (3' - 4' high)?

Also, my 26' x 51' house has a reentrant corner measuring 5.5' x 7.5',
respectively. The 5.5' exterior wall line extends into the house
3.5', so it is actually a 9' wall line continuously supported by the
foundation. In the short dimension, how should the base shear be
divided among the 26' wall line at one end, and the 20.5' wall line
with offset 9' wall line at the other end?

Thanks, Wayne

Bob Morrison

2005-12-12, 9:21 pm

In a previous post Wayne Whitney wrote...
> A few additional questions: with the partial basement, is the top of
> the concrete retaining wall (6"-12" above grade) considered the base?
> I'd like to have several windows in the cripple walls around the
> basement, which reduces the possible length of braced panels. Are
> there any preengineered products like Simpson Strong Wall and Simpson
> Steel Strong Wall which are suitable for cripple walls (3' - 4' high)?

No. But, APA does have test data on "high capacity" shear walls. Loads
as high as 1200 plf are possible, but require special construction
detailing.

>
> Also, my 26' x 51' house has a reentrant corner measuring 5.5' x 7.5',
> respectively. The 5.5' exterior wall line extends into the house
> 3.5', so it is actually a 9' wall line continuously supported by the
> foundation. In the short dimension, how should the base shear be
> divided among the 26' wall line at one end, and the 20.5' wall line
> with offset 9' wall line at the other end?
>

In that instance I usually ignore the re-entrant corner wall (9' long) and
dump all the load into the 20.5' wall. Otherwise you need a drag strut
and special detailing to get loads into the 9' wall. If you choose to use
the 9' wall, then load from half the diaphragm above will be applied to
it. In other words, you are going to apply half the load to a 9' long
wall instead of a 20.5' long wall. BTW, I would plywood sheet the 9' wall
anyway just for additional stiffness.

The above may not be strictly in accord with typical flexible diaphragm
analysis procedures, but it works. I would argue that since there is no
drag beam connected to the 9' wall it does not act as a shear wall.

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA

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